1. Field of the Invention
The invention relates to a solid state image pick-up device for photographing a still image or a motion image and outputting a still image signal or a motion image signal.
2. Related Background Art
Solid state image pick-up devices are mainly classified into a CCD sensor and a MOS type sensor. Generally, while the CCD sensor is excellent in its small amount of noises, it has such a drawback that electric power consumption is large. On the other hand, despite such an advantage of the MOS type sensor that its electric power consumption is much smaller than that of the CCD sensor, it generally has such a drawback that a noise amount is slightly large. However, there is a tendency toward a decrease in the noise amount in the MOS type sensor and it is expected that the MOS type sensor provides performance equivalent to or better than that of the CCD sensor in the future.
In the MOS type sensor, it is relatively easy to build in various functional circuits using MOS transistors. An amplifying type solid state image pick-up device having signal amplifying elements in pixels can be mentioned as such an example. In such an image pick-up device, there is liability to occurrence of noises called fixed pattern noises (FPN) which are caused by a variation in photodetectors and the amplifying elements constructing the pixels. As one of methods of correcting the fixed pattern noises, there is one whereby both of a dark state output (Vdark) and a light state output (Vsig) are read out from each pixel and a difference between them is used as a signal output. According to such a method, generally, two horizontal reading lines one for a dark signal and one for a light signal are necessary. Generally, the pixels of the odd-number designated pixels and the even-number designated pixels in the horizontal direction are simultaneously read out as Channel 1 and Channel 2, respectively, for the realization of a high processing speed. Therefore, in the case of using the lines for the dark signal (Vdark) and the lines for the light signal (Vsig), respectively, a total of four horizontal reading lines are needed.
FIG. 7 of JP-A-09-246517 shows an example of a circuit construction near horizontal reading lines of such an amplifying type solid state image pick-up device. The amplifying type solid state image pick-up device comprises: a pixel matrix 1 constructed by arranging amplifying type photoelectric converting elements in the row and column directions; horizontal reading lines 3 having, for example, a total of four wirings; a horizontal reading switch group 5; a capacitor group 7; a signal transfer switch group 9; a horizontal shift register 11; and an output amplifier group 12.
FIG. 5 is a diagram showing a schematic construction of the general MOS type sensor. The MOS type sensor comprises: a sensor array 100 in which a plurality of photoelectric converting elements 110 are two-dimensionally arranged; a vertical shift register circuit 120 for sequentially selecting rows of the photoelectric converting elements 110 from the sensor array 100; a line memory circuit 130 including signal charge holding capacities Cts for holding signal charges (S) serving as a photosignal and reset level (N) holding capacities Ctn for holding a reset level serving as a noise signal, respectively, from the photoelectric converting elements 110 of the selected row; a horizontal shift register circuit 140 for simultaneously selecting every two signal data among the signal data of one row held in the line memory circuit 130 by a transfer switch and transferring the selected signal data to a first photosignal side common output line (hereinafter, referred to as a first S output line) 210 and a first noise signal side common output line (hereinafter, referred to as a first N output line) 220 and also to a second photosignal side common output line (hereinafter, referred to as a second S output line) 230 and a second noise signal side common output line (hereinafter, referred to as a second N output line) 240; and first and second difference signal (S-N) reading circuits 150 for amplifying a first difference signal between the photosignal from the first S output line 210 and the noise signal from the first N output line 220 and a second difference signal between the photosignal from the second S output line 230 and the noise signal from the second N output line 240 and outputting the amplified signals, respectively.
The first difference signal is outputted from an output terminal (out1) 170 of the first difference signal reading circuit. The second difference signal is outputted from an output terminal (out2) 180 of the second difference signal reading circuit. The first S output line 210, the first N output line 220, the second S output line 230, and the second N output line 240 construct a common output line 160.
A wiring shape of a cross section near a cutting line 6-6 in FIG. 5 is as schematically shown in FIG. 6 in the conventional device. A cross sectional view of the first S output line 210, the first N output line 220, and their peripheries is shown here. As shown in FIG. 6, the first S output line 210 and the first N output line 220 are arranged in a same layer and a shielding line 901 is arranged between them. A shielding line 902 is arranged in a neighboring position of the left side of the first S output line 210. A shielding line 903 is arranged in a neighboring position of the right side of the first N output line 220. Further, a shielding plate 904 is provided under a region that covers a range from the shielding line 902 to the shielding line 903. The first S output line 210, the first N output line 220, the shielding lines 901, 902, and 903, and the shielding plate 904 show a similar layout and cross sectional shape wherever in a range from the left edge to the right edge of the common output line 160 they are arranged. Although not shown, a substrate is arranged under the shielding plate through an insulating layer. The substrate is generally a silicon substrate for forming a photoelectric converting portion.
As shown in FIG. 6, in the case where the shielding lines 901, 902, and 903, and the shielding plate 904 are provided, as shown at reference numeral 911, an electric line of force 911 which passes through a portion over the shielding line 901 is formed. Crosstalks are generated between the first S output line 210 and the first N output line 220 through the electric line of force 911. When the crosstalks are generated, a signal-to-noise ratio in an output of the S-N reading circuit 150 deteriorates.
Therefore, as described in David Johns, Ken Martin, “Analog Integrated Circuit Design”, John Wiley & Sons, Inc., 1997, as shown in FIG. 7, there is a method whereby the whole periphery of the first S output line 210 (or the second S output line 230) and the whole periphery of the first N output line 220 (or the second N output line 240) are covered by a shielding plate 953.
However, in the construction shown in FIG. 7, the signal flowing in the first S output line 210 (or the second S output line 230) is attenuated by a coupling capacity between the first S output line 210 (or the second S output line 230) and the shielding plate 953. Similarly, the noises which pass through the first N output line 220 (or the second N output line 240) are attenuated by a coupling capacity between the first N output line 220 (or the second N output line 240) and the shielding plate 953. Since the first S output line 210 (or the second S output line 230) and the first N output line 220 (or the second N output line 240) extend from the left end to the right end of the image horizontal direction of the solid state image pick-up device, the coupling capacity is distributed over a long distance. A slight increase in coupling capacity per unit length largely increases the whole coupling capacity and a degree of attenuation due to it is remarkable. This is because the signal charges read out of the pixel are temporarily accumulated in the capacities Cts and Ctn and, thereafter, read out to the first S output line 210 (or the second S output line 230) and the first N output line 220 (or the second N output line 240). Assuming that parasitic capacitances at this time are set to Chs and Chn, the read-out charges are subjected to capacity division between Cts and Chs and between Ctn and Chn and attenuated to Cts/(Cts+Chs) time and Ctn/(Ctn+Chn) time, respectively.
Therefore, in the case where the user intends to obtain an output of a desired level from the output terminal 170 (or the output terminal 180) of the S-N reading circuit 150, an amplification factor of the S-N reading circuit 150 has to be raised. A level of noises which are generated in the S-N reading circuit 150 rises at the output terminal 170 (or the output terminal 180) of the S-N reading circuit 150. Thus, the signal-to-noise ratio at the output terminal 170 (or the output terminal 180) of the S-N reading circuit 150 deteriorates.
Inherently, the signal and noises generated in the photoelectric converting element 110 appear in the first S output line 210 (or the second S output line 230) and the noises generated in the same photoelectric converting element 110 appear in the first N output line 220 (or the second N output line 240). The noises appearing in the first S output line 210 (or the second S output line 230) and the noises appearing in the first N output line 220 (or the second N output line 240) are cancelled in the S-N reading circuit 150. Only the signal between the signal and noises generated in the photoelectric converting element 110 can be obtained at the output terminal 170 (or the output terminal 180) of the S-N reading circuit 150. However, if the coupling capacity between the first S output line 210 (or the second S output line 230) and the shielding line differs from that between the first N output line 220 (or the second N output line 240) and the shielding line, the level of the noises in an input of the S-N reading circuit 150 in the first S output line 210 (or the second S output line 230) differs from that in the first N output line 220 (or the second N output line 240). The noises appearing in the first S output line 210 (or the second S output line 230) and the noises appearing in the first N output line 220 (or the second N output line 240) cannot be cancelled in the S-N reading circuit 150. It is impossible to obtain only the signal between the signal and noises generated in the photoelectric converting element 110 at the output terminal 170 (or the output terminal 180) of the S-N reading circuit 150.